66
J. N. BALL AND BRIDGET I. BAICER
proved difficult to follow with the light microscope, but they have been
traced between cells of the eel proximal pars distalis (Stutinsky, 1953)
and the trout pars intermedia (Billenstien, 1963), and pass toward the
cells of the proximal pars distalis in Subelinus (Honma and Tamura,
1965b).
At the electron microscope level, these nonstainable fibers appear to
correspond to the Type B fibers in the eel neurohypophysis, which contain granules of less than 100 mp diam, the granules having a central
electron dense material which does not fill the bounding membrane and
is less osmiophilic than the Type A granules (Knowles and Vollrath,
1966a,b). Similar granules are found in the NLT of eel (Knowles and
Vollrath, l W b , 1966b). In Sulmo, however, granule size in the Type B
fibers is less than in the NLT, so that Follenius (l%Sa,b) was not certain
that the NLT is the origin of the Type B fibers in this species. Moreover,
in some fishes (e.g., Phorinus) the NLT is said to be absent (Charlton,
1932), although Type B fibers are present (Follenius, 1965a,b). It is not
certain, therefore, that all Type B fibers arise from the NLT in all teleosts.
Nonstainable and Type B fibers appear to be more numerous in the
anterior part of the neurohypophysial core and in the pars distalis than
in the more posterior regions and the pars intermedia, although a few
Type B fibers are found in the eel pars intermedia (Knowles and Vollrath,
1966a), just as some type A fibers are found in the pars distalis (see Fig.
13 in Bern and Knowles, 1966). Type B fibers make both neurovascular
and neuroglandular contacts in both the pars intermedia and the pars
distalis. In the eel, the contacts in the pars distalis are all neurovascular;
that is, the neurons discharge into an extravascular space, close to the
endocrine cells but separated by a distance of about 300 m p or less
(Knowles and Vollrath, 1966b; see Fig. 23). However, in Hippocumpzcs,
both Type A and Type B fibers were found to make direct contact with
several kinds of endocrine cells in both regions of the adenohypophysis.
The contacts were synaptoid for both fiber types in the pars intermedia, and Type B fibers were shown to make synaptoid contact with
three types of pars distalis cells; but satisfactory evidence of synaptoid
contacts between Type A fibers and pars distalis cells could not be obtained (Knowles et al., 1967; Vollrath, 1967). Direct contact between
both types of fibers and pars distalis cells have also been seen in Tilapia,
as well as terminations on the basement membrane adjacent to the endocrine cells ( Nishioka and Bern, 1967). These findings suggest that activity
of the adenohypophysial cells could be regulated by a direct dual neurosecretory innervation (Types A and B), with various kinds of contacts
between the nerve fibers and the endocrine cells (Knowles and Vollrath,
196%; Vollrath, 1967).
J. N. BALL AND BRIDGET I. BAICER
proved difficult to follow with the light microscope, but they have been
traced between cells of the eel proximal pars distalis (Stutinsky, 1953)
and the trout pars intermedia (Billenstien, 1963), and pass toward the
cells of the proximal pars distalis in Subelinus (Honma and Tamura,
1965b).
At the electron microscope level, these nonstainable fibers appear to
correspond to the Type B fibers in the eel neurohypophysis, which contain granules of less than 100 mp diam, the granules having a central
electron dense material which does not fill the bounding membrane and
is less osmiophilic than the Type A granules (Knowles and Vollrath,
1966a,b). Similar granules are found in the NLT of eel (Knowles and
Vollrath, l W b , 1966b). In Sulmo, however, granule size in the Type B
fibers is less than in the NLT, so that Follenius (l%Sa,b) was not certain
that the NLT is the origin of the Type B fibers in this species. Moreover,
in some fishes (e.g., Phorinus) the NLT is said to be absent (Charlton,
1932), although Type B fibers are present (Follenius, 1965a,b). It is not
certain, therefore, that all Type B fibers arise from the NLT in all teleosts.
Nonstainable and Type B fibers appear to be more numerous in the
anterior part of the neurohypophysial core and in the pars distalis than
in the more posterior regions and the pars intermedia, although a few
Type B fibers are found in the eel pars intermedia (Knowles and Vollrath,
1966a), just as some type A fibers are found in the pars distalis (see Fig.
13 in Bern and Knowles, 1966). Type B fibers make both neurovascular
and neuroglandular contacts in both the pars intermedia and the pars
distalis. In the eel, the contacts in the pars distalis are all neurovascular;
that is, the neurons discharge into an extravascular space, close to the
endocrine cells but separated by a distance of about 300 m p or less
(Knowles and Vollrath, 1966b; see Fig. 23). However, in Hippocumpzcs,
both Type A and Type B fibers were found to make direct contact with
several kinds of endocrine cells in both regions of the adenohypophysis.
The contacts were synaptoid for both fiber types in the pars intermedia, and Type B fibers were shown to make synaptoid contact with
three types of pars distalis cells; but satisfactory evidence of synaptoid
contacts between Type A fibers and pars distalis cells could not be obtained (Knowles et al., 1967; Vollrath, 1967). Direct contact between
both types of fibers and pars distalis cells have also been seen in Tilapia,
as well as terminations on the basement membrane adjacent to the endocrine cells ( Nishioka and Bern, 1967). These findings suggest that activity
of the adenohypophysial cells could be regulated by a direct dual neurosecretory innervation (Types A and B), with various kinds of contacts
between the nerve fibers and the endocrine cells (Knowles and Vollrath,
196%; Vollrath, 1967).